1. Field of the Invention
The present invention is directed to the field of fuel supply systems for engines. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the provision of a combustible air/fuel mixture to an internal combustion engine. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the provision of a fuel system including means for vaporizing a liquid fuel to form a fuel vapor which may be mixed thereafter with atmospheric air to generate a combustible air/fuel mixture. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the provision of a combustible air/fuel mixture having a substantially constant air/fuel ratio and which minimizes the tendency of fuel vapor to condense or collect prior to entry of the combustible mixture into the combustion chambers of the engine.
2. Description of the Prior Art
The internal combustion engine most frequently encountered in current technology is powered by a liquid fuel which is introduced into the combustion chambers of the engine in conjunction with a quantity of atmospheric air. This mixture is subsequently ignited by compression or by an electrical spark. The quantity of fuel introduced into the engine is coarsely calculated to provide a combustible mixture for each engine combustion chamber and to provide, upon combustion, sufficient energy to achieve a desired operating characteristic for the engine. While the air/fuel ratio of the total combustible mixture may be relatively closely controlled, it is known that cylinder-to-cylinder variations in the air/fuel mixture can be significant.
The most frequently encountered air and fuel delivery systems for internal combustion engines involve the metering of a quantity of liquid fuel for mixture with an air stream to form, upstream of the combustion chambers of the engine, a combustible mixture of air and fuel. The most commonly encountered system allows liquid fuel to be delivered under a pressure differential directly to an air stream substantially upstream from the combustion chambers. Turbulence in the air stream is allowed to accomplish mixing. A variety of schemes have been suggested in the prior art to provide uniform distribution of the mass of fuel throughout the mass of air. For example, the prior art shows a variety of techniques to "atomize" or otherwise finely divide the liquid fuel droplets.
As an alternate to delivery of the liquid fuel upstream from the combustion chambers, it is not uncommon for a quantity of liquid fuel to be delivered directly to the combustion chamber with the amount of fuel being calculated to provide approximately stoichiometric combustion with the quantity of air ingested by the engine under the then-existing operating conditions of the engine. The more recently introduced electronic fuel injection provides quantities of fuel in proximity to, but upstream from, the intake port of the combustion chamber for ingestion by the combustion chamber with a quantity of air provided in the intake manifold. A less common technique is to deliver a quantity of liquid fuel into the combustion chamber and to allow turbulence and heat therein to mix the fuel with the ingested air.
As thus described, virtually all air and fuel metering and mixing systems for internal combustion engines rely on mixing a measured quantity of fuel, in liquid form and typically a petroleum product such as gasoline or diesel fuel oil, with a quantity of atmospheric air in gaseous form for introduction into the combustion chambers of the engine. It has long been understood that the quantity of fuel be approximately uniformly distributed throughout the air mass in order to promote good combustion. In order to achieve this result, a large number of schemes have been recommended in the prior art to atomize or otherwise finely divide and distribute the liquid fuel within the air stream. However, as will be appreciated, the uniform distribution of a large number of droplets of liquid within an air stream is virtually impossible. For example, the walls of the intake manifold tend to accumulate liquid droplets (become wet) and the quantity of fuel so attached to the manifold walls may vary. A further problem results from the different mixture flow paths between the point of mixing of the air/fuel mixture and the different combustion chambers. As a result of the above-noted facts (1) a transient time delay may occur before changes in the rate of fuel delivery to the air stream may produce a change in the rate of fuel delivery to the combustion chambers as fuel is accumulated, or given up, by the manifold walls and (2) cylinder-to-cylinder air/fuel ratios may vary significantly as the relatively heavy liquid fuel droplets are nonuniformly distributed at mixture flow junctions.
It has been proposed that uniform distribution of a quantity of liquid fuel within a moving air stream may be achieved by converting the quantity of liquid fuel into a vapor, as by heating. It is therefore one object of the present invention to provide a fuel delivery and metering system for use with an internal combustion engine which system is capable of providing the engine with a combustible mixture having vaporized liquid fuel. In order to avoid needless complexity, it is a further object of the present invention to provide such a fuel delivery and metering system which vaporizes the liquid fuel prior to the mixing of the fuel with air. It is a still further object of the present invention to provide such a system which includes means for metering a vaporized liquid fuel.
In providing such a fuel delivery and metering system, a variety of problems are encountered. For example, commercially available gasoline is normally completely vaporized, at atmospheric pressure, at temperatures of about 425.degree. F. In order to maintain the fuel in vapor form prior to metering and mixing, it is necessary to minimize the contact between such vaporized fuel and any surfaces which could promote condensation. However, once the vaporized fuel is mixed with air for delivery to the engine, some contact with lower temperature engine components can be anticipated. It is therefore an object of the present invention to provide such a system which inhibits the contact of the vaporized liquid fuel with any structural component surfaces which could promote condensation.
The use of a vaporized liquid fuel for mixture with atmospheric air to provide a combustible air/fuel ratio has previously been proposed. However, the prior proposals for providing such a fuel delivery and metering system have usually incorporated additional structure which, in turn, has generated substantial technical problems particularly in application of such a system to an automotive internal combustion engine. For example, it has been proposed that the liquid fuel be vaporized by situating an electrical heater within an otherwise conventional liquid fuel delivery conduit without otherwise adjusting for the volume change in vaporization of liquid fuel. Such systems have suggested metering based on liquid fuel delivery. In such a system, the air/fuel ratio of the combustible mixture so produced may be expected to vary (1) as a function of fuel delivery through the conduit upstream from the heater means, (2) as a function of atmospheric pressure, (3) as a function of the flow rate of air within the air delivery conduit, (4) as a function of the composition of the liquid fuel and (5) as a function of the long term performance of the metering apparatus.
Other suggestions for using a vaporized liquid fuel have included the provision of liquid fuel to a carburetor or other metering device for various phases of engine operation. Such systems are, of course, unsatisfactory in that the problems attendant to liquid fuel systems generally will occur during those modes of engine operation where fuel delivery in the liquid state is necessary. It is therefore a specific object of the present invention to provide a vaporized liquid fuel delivery system which operates with a liquid fuel in vapor form for steady state and transient operation of the associated internal combustion engine. More particularly still, it is a specific object of the present invention to provide such a fuel delivery and metering system which will deliver vaporized liquid fuel during, and will accommodate, all modes of engine operation particularly when that engine is intended to power a motor vehicle. With the foregoing objective in mind, it is a further and particular objective of the present invention to provide a fuel delivery and metering system adapted to use a vaporized liquid fuel which system substantially reduces the likelihood of vaporized liquid fuel from condensing under circumstances tending to permit liquid fuel to enter the intake system of an associated engine. It is a further and particular object of the present invention to provide such a fuel delivery and metering system which avoids the accumulation of relatively nonvolatile heavy fuel fractions under conditions which would tend to prevent an adequate supply of vaporized liquid fuel for normal engine operation.
In designing a fuel delivery and metering system to accomplish the foregoing objectives, it is necessary to consider a variety of technical problems. In powering a motor vehicle, an internal combustion engine will typically consume an air mass under maximum consumption operating conditions which is approximately (20) times the mass of air consumed by the engine during its minimum consumption operating conditions. In order to provide for a substantially constant air/fuel ratio over the entire range of engine operation, it is necessary that the fuel delivery and metering system be capable of providing quantities of fuel which may range over a ratio of mass flow rates of (20:1). Thus, in designing the fuel delivery portion of a vaporized liquid fuel delivery and metering system it is necessary to be able to vaporize a mass flow rate of fuel at maximum engine operating conditions which will be (20) times the mass flow rate of fuel necessary for minimum operating conditions of the engine. While it is possible to achieve this ratio of vaporization of a mass flow rate of liquid fuel by modulating the flow through a conduit which includes a vaporizing means, it will be appreciated that such an approach, in order to provide a substantially constant air/fuel ratio, would require sophisticated techniques of measuring air flow and sophisticated techniques of monitoring and modulating the flow of the liquid fuel into the vaporizing means. It is therefore a specific object of the present invention to provide a vaporized liquid fuel delivery and metering system which is capable of vaporizing quantities of fuel commensurate with maximum engine operation and which may readily provide quantities of vaporized fuel consistent with minimum engine operation. It is a still further object of the present invention to provide such a system which bases fuel metering on metering of the vaporized liquid fuel.
The prior art has also suggested that the vaporized liquid fuel be provided to the air stream under superatmospheric pressure as, for example, by a positive displacement pump. Such systems would have pressure being varied to accomplish variation in fuel delivery rates. In view of the wide variety of atmospheric conditions under which a motor vehicle having an internal combustion engine is intended to operate, such a system requires means to compensate the vaporized fuel pressure in view of existing atmospheric pressure. This adds unnecessary cost and complexity to any kind of fuel system. It is therefore an object of the present invention to provide a vaporized liquid fuel delivery and metering system which does not rely on pressures in excess of atmospheric pressure to deliver and meter the fuel to the air stream. It is therefore a further object of the present invention to provide a vaporized liquid fuel delivery and metering system which may maintain a substantially constant air/fuel ratio by automatically compensating fuel pressure for atmospheric pressure variations without the addition of pressure sensing devices or of pump modulation or control devices. More particularly still, it is an object of the present invention to provide such a system in which the vaporized liquid fuel is maintained at substantially atmospheric pressure. More particularly still, it is an object of the present invention to provide a vaporized liquid fuel delivery and metering system in which vaporized liquid fuel is delivered to the air stream for mixture therewith at a vapor pressure upstream from the vapor delivery nozzle which is substantially equal to the pressure of the air source immediately upstream from the engine air intake under substantially all engine operating conditions. It is a specific object of the present invention to provide a vaporized liquid fuel delivery and metering system which meters vaporized liquid fuel to an air stream to establish an accurately controlled and maintained air/fuel ratio over a wide range of mass flow rates. It is a still further and particular object of the present invention to provide a fuel delivery and metering system for delivery of a vaporized liquid fuel to an air stream to form a combustible air/fuel mixture having a substantially constant air-to-fuel ratio for mass air flows which may vary rapidly by a factor of (20:1). It is a further object of the present invention to provide a mechanism for modulating the quantity of vaporized liquid fuel delivered by such a system. More particularly still, it is a further and specific object of the present invention to provide a vaporized liquid fuel delivery and metering system which may be made responsive to changes in the partial pressure of oxygen in the combustion by-products produced by the combustible mixture for maintaining the combustible mixture at a substantially constant air/fuel ratio.